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swiftly; rapidly; quickly; immediately; hastily; hurriedly
suddenly; abruptly; unexpectedly
sharply (of a slope, bend, etc.); steeply
急に走ったので息が切れて足も痛くなった。
I ran so fast that I got out of breath and my legs hurt.
急に空が暗くなって雨が降り始めました。
It suddenly started raining.
急に対応しなければ間に合わない。
If we don't respond quickly, we won't make it.